It is well known among metallurgists that cast iron and other ferrous materials (i.e. iron and iron containing alloys) contain carbon as a result of the casting process. Upon cooling, the carbon precipitates out of solution and is present in the piece as graphite flakes. The size and distribution of the graphite flakes can vary as a function of the alloy, as well as of the parameters of the casting and cooling processes.
One process for bonding iron or ferrous pieces with principally aluminum pieces is the AlFin process. In the AlFin process, the bonding surface of the ferrous piece is immersed in molten aluminum in order to form an intermetallic Al--Fe bonding surface layer. The aluminum piece is then bonded to the ferrous piece at the Al--Fe intermetallic layer.
The AlFin process is commonly used, for example, to bond ferrous alloy piston ring carriers to aluminum pistons, such as those used in diesel engines. The bonds must be strong enough to withstand the stresses generated by the elevated temperatures and pressures produced during the operation of a diesel engine. If the cohesion is weak, debonding may occur. The problem of debonding is especially troublesome if the debonding should happen while the engine is in service. Therefore, one limitation generally associated with the prior AlFin process is debonding under prolonged exposure to high pressure/temperature environments and debonding under acute exposure to extreme pressure/temperature environments.
Hence, there is a need for an improved ferrous-to-nonferrous metal bonding process (and in particular an improved iron-to-aluminum bonding process) capable of producing intermetallic bonds that are stronger and more resistant to harsh environments. In the case of aluminum pistons used in diesel engines, the intermetallic bond between the piston and piston ring carrier must be able to withstand prolonged exposure to the high pressure/temperature environment of the engine as well as acute exposures to pressure and temperature extremes without debonding. A means for satisfying this need has so far eluded those skilled in the art.